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Cleaning Audio Background Noise


This article was first published by PC World in December 2008.

Sometimes the recordings you make just don’t turn out as well as you would have liked. There may be background hiss, clicks, pops or distortion present in the recorded audio, or you may simply be trying to record something in a hostile environment such as a busy street, an airport or even on a noisy telephone line.

Other times you inherit recordings that are noisy and have to work with the consequences. If at all possible it’s best to try to record the piece again, but often this isn’t an option, so you need to switch to rescue mode and start looking for a way to salvage the contents of the audio file.

Thankfully today’s sound-editing software tends to have quite a comprehensive range of tools that you can use to remove various background noises. These range from easy-to-use plug-ins such as noise gates to much more complex noise-reduction tools that take a digital footprint of the background noise and then use complex algorithms to try to separate it from the main audio signal.

But while noise-reduction tools have become much more impressive over the years, you still have to be realistic about their abilities. You’ve heard the saying that you can’t turn a pig’s ear into a silk purse, and the same holds true for noise reduction.

If the signal is atrociously bad in the first instance, no amount of tweaking is going to turn it into a pristine audio file. You may have seen noise-reduction tools work wonders on TV shows such as CSI, taking a barely comprehensible audio track and making it sound as if it was captured in a recording studio, but in real life this never happens.

You can often remove significant amounts of hiss, hum, crackle and pops, but the worse the original signal is, the less likely you are to get good results in the final processed signal. But that’s not to say noise reduction is a lost cause. Often it can work wonders – ­ you just need to be realistic about what’s possible before starting out.

The spectrum of noise
What makes noise reduction such a tricky process is that noise usually comprises lots of different types of sounds, with different frequencies and timbres all mixed together. Often it hops around the frequency spectrum, jumping from low- to high-frequency sounds and everything in between.

In this column we’re going to investigate some of the ways in which you can use noise-removal tools to clean up your recordings. Our examples will be based on the noise-removal tools available in V1.3.5 of Audacity, which can be downloaded free.

However, the methods we’re looking at can be easily transferred to other packages, such as Adobe’s Audition and Sony’s Sound Forge.

Before using the noise-reduction tools we first need to make sure our audio file is in its best possible state. There are two processes you should always perform on audio files before starting any processing. The first is to remove any DC offset and the second is to Normalise the file.

If a file is suffering from DC offset, the mean amplitude of its waveform will not be set to zero. As a result, when you go to normalise the file it will not increase to its maximum amplitude. Normalising increases the amplitude of the whole audio file to its maximum level without introducing any distortion.

Your file may initially sound worse, because normalisation increases the whole amplitude of the file, including the background noise. However, using normalisation at the end of the noise-reduction process may pull out background noise you had previously missed.

Load your file in Audacity and then click on the Effect menu and select Nomalize. Make sure both the ‘Remove Any DC Offset’ and ‘Normalize Maximum Amplitude’ boxes are ticked and then click OK.

The golden rule of noise reduction is to try to apply as little processing as possible to the signal, because the more aggressive you are with the noise-reduction controls, the more artefacts from the noise-reduction process itself will come through in the final results.

Using Audacity
Audacity’s Noise Removal tool can be confusing for beginners, as it doesn’t come with any instructions. The main hurdle is knowing how to tell the tool to lift a noise profile for your audio file.

First you need to ignore the noise-reduction tool completely. Instead, scan through your file looking for a place where only the background noise can be heard. Then you need to use the selection tool to highlight a section of it that’s less than half a second in length.

In the file in the screenshots there’s a pause between words where we can pick out the background noise. Once we’ve highlighted this section we then go to the Effect menu and select Noise Removal. When the effect’s interface appears click on the Get Noise Profile button.

The Noise Removal tool will disappear from the screen. Now you need to select the portion of the track you want to apply the noise reduction to. Usually this will be all of the track so you can just hit Ctrl & A to select the entire file. Click on the Effect menu again and select Noise Removal.

Click on the Preview button to listen to how the file will sound with noise reduction applied. If you can still hear background noise pull the Noise Reduction slider to the right to increase the aggressiveness of the reduction process.

Try making small adjustments, as you want to remove as much noise as possible without negatively affecting the audio quality of the final file.

Adjusting the main slider on its own may not be enough. In which case try playing with the Frequency Smoothing and the Attack/Delay Time controls. It’s best to start with them set at around the halfway mark.

You’ll find that moving them to the left makes the noise removal more aggressive, but is also likely to introduce more artefacts, while moving them to the right will reduce the number of artefacts, but also reduce the aggressiveness of the noise-reduction process.

When you’ve got a good balance between the amount of noise reduction and the quality of the file, click on the OK button to apply the changes.

Audacity also includes a Click Removal tool, useful for removing clicks from recordings that you’ve made of albums on vinyl. Load your recording into Audacity and click on the Effect menu and select Click Removal from the dropdown menu. The tool has two sliders.

The first is marked Select Threshold and dictates the amplitude at which the effect will recognise a piece of audio as being a click, rather than something it should ignore. The second control, Max Spike Width, is used to set the length of spiked audio to be considered a click.

The higher the values you use on these two sliders, the more aggressive the effect. Play around with the controls and use the preview button to hear the effect of the changes you make.

Don’t be overly aggressive with the controls, as it can have a significant effect on the final quality of the music. Once you’ve got a setting you’re happy with, click on OK to write the changes to the file.

If there are just one or two clicks that need to be removed, you could try doing it manually using Audacity’s pencil tool. Find the spike in your waveform and then select the zoom tool. Keep zooming in on the waveform until it changes to show individual samples marked with cross hairs.

Now switch to the pencil tool and draw over the spike with a smoother waveform. Hit Play and you should find that the click has gone.

Using the combination of the Noise Removal and Click Removal tools you should be able to remove background hiss, hum and clicks. And with a little bit of patience and experimentation you may even be able to remove really stubborn noise and salvage recordings that would otherwise be destined for your PC’s recycle bin.

Using noise gates
One of the oldest methods of removing noise from a recording is to use a noise gate. Analogue noise gates have been around for years and were commonly used to reduce the background hum and hiss of amps during quieter parts of a recording. Of course, pretty much every piece of recording software now includes a noise gate effect.

A noise gate is a pretty easy device to use. You simply set a threshold of a minimum level that the audio must reach before it can pass through the gate. If the threshold is not reached no sound is heard, but once the sound level breaks through the threshold the noise gate opens and lets all the audio through.

Noise gates are very good for reducing hiss and hum during gaps in a recording, but once the gate is open they don’t reduce any background noise at all, so they’re no use for general noise reduction.